1. Field of the Invention
The invention relates to a catadioptric objective and more specifically it relates to a projection objective for semiconductor lithography comprising a beam splitter surrounded at least partially by a mount, comprising a mirror, comprising a plurality of lenses and comprising a xcex/4 plate arranged between the mirror and the beam splitter.
Such an objective is described in DE 44 17 489 A1.
2. Description of the Related Art
The aim in theory is for no false light to be produced in the case of an objective of this type having a polarization beam splitter. It is known from the document mentioned at the beginning to direct incident s-polarized light in the beam path of the objective onto a beam splitter. In this case, 100% of the s-polarized light is reflected, subsequently rotated with the aid of a xcex/4 plate and then reflected onto a mirror. The light reflected at the mirror strikes the beam splitter again and then passes in transmission, as p-polarized light, through the beam splitter in the direction of the image, for example a wafer, to be exposed.
In practice, however, the light passing through the beam splitter is not perfectly p-polarized, since all the optical elements in the objective exhibit stress birefringence. Moreover, in practice a beam splitter generally has a reflecting power of only 80% for the incident light. The light not reflected therefore passes through the beam splitter, strikes the mount situated therebehind and is then reflected to and fro in the objective until it subsequently strikes the object to be exposed, for example the wafer, as interfering scattered light
Reference may be made in addition to U.S. Pat. No. 5,072,126 in relation to the general prior art.
It is the object of the present invention to create an objective structure that prevents interfering scattered light from being produced, in particular scattered light that strikes the object to be exposed.
This object is achieved according to the invention by means of a catadioptric objective, wherein arranged between the beam splitter and the mount on the side on which false light emerges from the beam splitter is an absorption device that absorbs the false light traversing the beam splitter.
According to the invention, there is now created for the portion of the incident light that is not reflected at the beam splitter but passes through the latter a type of beam trap in the form of an absorption device which prevents scattered light being formed by light reflected at the mount.
Fundamentally, the absorption device according Lo the invention can also be achieved by means of an appropriate design of the mount such as, for example, an antireflection layer and/or blackening at the region of the mount that is located on the side of the beam splitter opposite the incident light if the heating caused thereby can be mastered. This holds, for example, for a stainless steel surface, which yields a serious lowering of the reflection in the event of a coating, in particular in the wavelength ranges of 248, 193, 157 or 126 nm.
In general, tho false light will emerge on the side that is opposite the ray bundle incident in the beam splitter. However, it is also possible for false light to emerge on another side. All that is required in this case is also to provide an absorption device on the corresponding side.
In another advantageous refinement of the invention, the absorption device can be formed by a glass substrate. If, in this case, a glass substrate is selected which absorbs the light after a few millimeters, a space-saving design is created in this way which also does not produce temperature problems. This is the case, in particular, whenever the glass substrate is brought into a mechanical contact with the mount that transfers heat. The mount can then effectively dissipate the heat produced during the absorption, in particular when the mount is temperature-stabilized, that is to say provided with a heat dissipating means.
However, instead of a glass substrate it is also possible to arrange another absorption device between the beam splitter and the mount which is likewise, if appropriate, in mechanical contact with the mount that transfers heat.
In order for the beam trap to function satisfactorily, it is advantageous when the beam splitter is coated on the side averted from the beam input side so that reflection does not occur on the rear wall of the beam splitter itself.
Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of exemplary embodiments of the present invention.
The invention is explained below by way of example for a catadioptric projection objective for semi-conductor lithography. Since such an objective is generally known (see, for example, DE 44 17 489 A1), only the parts essential to the invention are described below in more detail.